Heretofore, a catalyst system comprising titanium trichloride and an organoaluminum compound has been used in the commercial production of a highly crystalline polymer of an .alpha.-olefin such as propylene or butene-1.
Since the polymerization in this production proceeds in a form having solid titanium trichloride embraced in the polymer being formed, titanium trichloride remains in the resulting polymer. This residual titanium trichloride adversely affects such properties of the polymer as hue and thermal stability and hence, the polymer generally has been required to undergo a treatment capable of removing the polymer of the residual titanium trichloride through extraction. Besides, the polymerization forms as a by-product an amorphous polymer called an atactic polymer. When this atactic polymer is suffered to mingle into the produced polymer, it degrades the mechanical properties of the polymer or imparts viscidity to the polymer. In this respect, the polymer has been required to be given a treatment capable of removing the atactic polymer by washing.
The necessity for such extra treatments has entailed an economic disadvantage in terms of natural resources and energy. Thus, simplification of the conventional polymerization has been earnestly in demand. Various improvements have been given to date to the method for the production of titanium trichloride. Consequently, the recent products of this method exhibit notably improved polymerization activity and/or stereoregularity. Thus, a polymerization process which obviates the necessity for the aforementioned extra treatments has become commercially feasible. As one version of the method for the production of titanium trichloride, some of the present inventors formerly proposed a method disclosed in Japanese Patent Publication No. 27085/80. As an improvement over this version, they also proposed a method disclosed in Japanese Patent Application (OPI) No. 116706/81 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application").
When a solid product obtained by reducing titanium tetrachloride with an organoaluminum compound or a product obtained by thermally treating the aforementioned solid reduction product is used as a titanium trichloride composition, and the composition is reacted with a halogen compound and an ether compound or with a halogen compound, an ether compound and an electron donor in any of the methods disclosed in Japanese Patent Publication No. 27085/80 and Japanese Patent Application (OPI) No. 116706/81, then the reaction product will experience great difficulty in passing through a filter during the course of the subsequent washing treatment. By this reason, the separation of the reaction product has had to rely on a decantation process which suffers from poor solid-liquid efficiency. When the reaction product is further dried subsequently to the washing treatment, tough masses occur in a fairly large volume. If these tough masses are directly put to use in the polymerization of an .alpha.-olefin, they may possibly clog the feed line for solid catalyst or they may give rise to solid polymers within the polymerization vessel and eventually entail the trouble of clogging the valve used for the discharge of the produced polymer. Thus, the dried reaction product containing such tough masses has had to be sifted for removal of such tough masses. These problems have been prominent during the working of the method disclosed in Japanese Patent Application (OPI) No. 116706/81.
As the result of extensive studies, the present inventors have found that in the methods disclosed in Japanese Patent Publication No. 27085/80 and Japanese Patent Application (OPI) No. 116706/81 comprising the steps of reacting a titanium trichloride composition with a halogen compound and an ether compound and, if desired, an electron donor in an aromatic hydrocarbon solvent, subjecting the reaction product to solid-liquid separation and then washing the separated solid product, the aforementioned problems can be solved by using an aliphatic hydrocarbon in at least the first of washing treatments so performed on the solid product mentioned above. This invention has issued from this knowledge.